1. Field of the Invention
This invention relates to an overcurrent protective circuit for a modulated-conductivity type MOSFET.
2. Description of Background
A modulated-conductivity type MOSFET (metal oxide semiconductor field-effect transistor) is a FET which is provided with a MOS gate input, operates in a bipolar mode, and has such advantages as rapid switching speed and low ON (saturation) voltage. This permits a high-power high-frequency control which has not been available by use of conventional bipolar transistors or MOSFETs, and allows compactness and low cost of various apparata to be realized. Hereinafter, the above-described modulated-conductivity type MOSFET is simply referred to as BIFET (Bipolar, mode FET).
FIG. 1 shows a basic chopper circuit of a BIFET, wherein reference numeral 1 designates a BIFET. In FIG. 1, the turn-on and turn-off operation of the BIFET 1 functions so as to supply power from a DC power source 2 to a load 3. The BIFET 1 is on-off controlled by a gate signal generating circuit 50 which has a gate power source 4 that supplies a positive voltage to the gate of the BIFET 1, a gate power source 5 that supplies a negative voltage to the same, and bipolar transistors 6 through 9 that amplify a control signal received at a control signal input terminal 10. When the control signal input terminal 10 of the gate signal generating circuit 50 receives a positive signal, the transistors 6 and 7 are turned on so as to supply a positive voltage from the gate power source 4 through an output terminal 11 to the gate of the BIFET 1, which is thereby turned on. When the control signal input terminal 10 receives a negative signal, the transistors 8 and 9 are turned on so as to supply a negative voltage from the gate power source 5 through the output terminal 11 to the gate of the BIFET 1, which is thereby turned off.
FIG. 2 is a graph illustrating one example of characteristics between a drain voltage V.sub.D and a drain current I.sub.D, both of a BIFET. As shown, when operated with higher gate voltages V.sub.G, ON voltages of the BIFET become lower, whereby power loss therein can be reduced.
In FIG. 1, when a short-circuit failure occurs within the load 3, the voltage between the drain and source of the BIFET 1 rises up to the voltage of the DC power source 2. As a result, power loss within the BIFET 1 becomes excessively large, thereby causing the BIFET 1 to be damaged. Should the BIFET 1 be operated with the gate voltages lower taking such a failure within the load 3 into consideration, as seen from FIG. 4, ON voltages of the BIFET 1 become higher, whereby the power loss within the BIFET during the ON state becomes larger.
To solve the above-mentioned problem, there is provided an overcurrent protective circuit shown in FIG. 3. In FIG. 3, between the drain and source of the BIFET 1, are connected resistors 12 and 13 in series, and a voltage between the drain and source is detected across the resistor 13. Between the gate and source of the BIFET 1, are connected a resistor 41 and a transistor 42 in series, and the base of the transistor 42 is connected through a zener diode 43 to the higher potential side of the resistor 13. The gate of the BIFET 1 is connected through a resistor 44 to the output terminal 11 of the gate signal generating circuit 50.
In operation, when a failure within the load 3 causes an overcurrent to flow through the BIFET 1, the ON voltage of the BIFET 1 rises. This ON voltage is divided by the resistors 12 and 13, and when the voltage across the resistor 13 exceeds the zener voltage value of the zener diode 43, a current flows into the base of the transistor 42. This causes the transistor 42 to be turned on, so that the voltage of the gate power source 4 becomes divided by the resistors 41 and 44 so as to be lowered. For example, assuming that the voltage of the gate power source 4 is 15 V and both the resistors 41 and 44 are 50 .OMEGA., the gate voltage of the BIFET 1 is 15 V when operated in normal operation. However, after a short-circuit failure has occurred within the load 3, the gate voltage is lowered to 7.5 V, whereby a current that flows through the BIFET 1 can be reduced.
On the other hand, when the BIFET 1 is turned on with the load 3 which is in normal state, there exists, at the initial turn-on period thereof, a delay time of several tens of nanoseconds. Thus, during the period of several tens of nanoseconds from an instant at which a positive gate voltage is applied to the gate of the BIFET 1, the voltage of the DC power source 2 is applied between the drain and source of the BIFET 1. During this period, a current flows into the base of the transistor 42, so that the gate voltage of the BIFET 1 becomes lower value. However, as time advances, the ON voltage of the BIFET 1 is gradually lowered, and ultimately reaches a value of several volts. Should a voltage developed across the resistor 13 at this instant become lower than the zener voltage of the zener diodes 43, the transistor 42 becomes turned off and the gate voltage of the BIFET 1 rises up to the voltage of the gate power source 4, so that the BIFET 1 can be operated such that the ON voltage thereof becomes sufficiently lowered.
FIG. 4 is a graph illustrating the relationship between a drain current I.sub.D (max) and a drain-source voltage V.sub.D of a BIFET in the case when the BIFET is damaged due to an overcurrent that flows thereinto. In FIG. 4, the hatched portion is a region in which the BIFET is damaged. As can be seen from the graph, I.sub.D (max) is in inverse proportion to V.sub.D, and it becomes significant that the overcurrent be reduced as low as possible particularly when the BIFET is utilized in a high voltage circuit. To achieve this, it is necessary that the gate voltage of the BIFET be restricted either below Vth (a minimum gate voltage to cause the BIFET to be in the ON-state) so as to cause the flow of current to cease, or below a value of approximately Vth+3 V so as to sufficiently reduce the current flow.
However, in the conventional protective circuit shown in FIG. 3, should the resistors 41 and 44 be determined such that the gate voltage of the BIFET 1 becomes less than or equal to Vth when an overcurrent flows into the BIFET 1, several problems are developed. First, as described above, at the initial turn-on period of the BIFET 1, the voltage of the DC power source 2 is applied between the drain and source of the BIFET 1, so that the transistor 42 is turned on, and at this instant the gate voltage of the BIFET 1 inevitably decreases to a level of less than or equal to Vth. As a result, the BIFET 1 does not become turned on, or the turn-on time thereof becomes extremely longer. Second, in the case when a failure within the load 3 causes the protective circuit to operate, an overcurrent that flows through the BIFET 1 decreases abruptly, so that a voltage applied to the BIFET 1 oscillates due to a stray inductance component of the circuit, and the voltage developed across the resistor 13 becomes temporarily lower than the zener voltage of the zener diode 43. At this instant, the transistor 42 becomes turned off, and a high gate voltage is applied again to the BIFET 1, thereby causing an overcurrent to flow. The above-described repetition induces oscillatory phenomena within the circuit.